Equalizing the tiedown positions after
removing the lead.
09dec07
The lead in the corners of the platform and the
laser shack were removed for the painting project. After the project we
decided to not replace these. The removed weight was about 20,000
lbs. We need to replace this weight with tiedown tension to level the
platform. This will give more pretension in the tiedowns (at
least 12 and 4). When the dome (at high za) passes in front of a
tiedown, this extra pretension will help not loose tension in the cable
so soon.
Tilt
sensor az swings were done and the 1 azimuth
term amplitude was measured. This tells how "uneven" the tiedown
positions are.
- The tiedowns were set to: [15.115,15.703,11.917] (td12,td4,td8)
to compensate for the missing corner weight.
We want to have all of the extensions be the same and to have the
tensions in the cables unchanged (so we are still balanced). We moved
all the tiedowns to the td12 position. This required the NUTS on the
cable Ubolts to be moved in the opposite direction so the tension
remained the same. The motions were:
- Nuts for td 8 up 3.25 inches
- Nuts for td4 down .5 inches
- NOTE: tiedown positions measure how far the tiedown pulls down
from the maximum extension(minimum tension).
We need to worry a little about the dynamic range:
tiedown parameters
tiedownMotion/PlatformMotion
|
1.66
|
degF per platform inch
|
5degF/platformInch
|
Kips/platformInch (no loss td
tension)
|
8.8Kips/platformInch
|
Kips/platformInch (add weight to
platform
and don't readjust td for lost tension).
|
24Kips/platformInch
|
- How low in temperature can we control the tiedown average height:
- we have jack extension: 17 inches at 70 degF and 156 kips total
on the platform (current values as i write this)
- tiedown pull down limit is 22 inches so we might operate to 21
inches in normal operation.
- (21"-17")/ (1.66 tdInch/5degF)*5degF=12 degF . So we can
operate down to 70-12=58 degrees F.
- How high in temperature can we control the average height of the
platform (ignoring tension loss from dome at high za).
- The lower limit of the tiedown extension is around 2 inches so
we might work to 3 inches.
- (17-3)/1.66*5=42 degF + 70 = 112 degF .. note this is the
average height computation and does not include tension loss caused by
the dome at high za.
- How much tension do we have when the tiedowns are completely
released (for
hurricanes and disconnecting cables):
- If we try and release the tiedowns at 70 deg F (probably a
pretty good temp for a wet hurricane) we will be left with:
- 156kips - (17-2)/(1.66 tdInPerPlatformIn) * 8.8kips/platform
inch = 76 kips. This is probably close to what we had before (including
the 20 kips that we removed from the platform and replaced with tiedown
tension).
- To release cable tension we usually do it at a higher temp. We
can also raise the dome up to 20 degrees and swing it in front of a
tiedown. (We released the tension in each cable to adjust the nuts with
the temp=70 degrees and wet without any problem).
- What happens with the tension release or over tension when the
dome passes in front of or opposite to a tiedown.
- A quick look on 08dec07 at 70 deg with the dome opposite td 4
had about 80 kips .. so we aren't going to lift the block of the ground.
- we need to look at this a bit more for the ight and low temps.
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